The present invention relates to a method and apparatus of monitoring the temperature and/or temperature profile of a material during heating and it will be describe with particular reference thereto; however, it will become appreciated that the invention has broader aspects in ascertaining the energy input into the material and the effects of different rates of heating the material with respect to the materials metallurgical properties.
For many years, there has been a demand for controlling heating systems by a variety of non-destructive sensors which could be interfaced with microprocessors to control and/or monitor the heating process and/or determine whether the workpiece has been inadequately heated or otherwise thermally processed and therefore is defective. One method has been to insert thermocouples into the material to measure the thermal change in the material during heating. This method has proven to be impractical for industrial application and is limited to laboratory testing. Thermocouples are typically imbedded into the material to be tested, thus are usually destroyed or are impractical or impossible to reuse once the material is heated.
A common method presently used in industry for controlling the heating of materials is the use of optical pyrometrics. The pyrometer optically measures the radiant energy from the material surface which in turn can only be converted into the surface temperature of the material. However, the accuracy of a pyrometer is affected by the surface conditions and resultant surface emissivity. Inaccurate readings will occur due to scaling of the material and/or incandescent particles within or on the surface of the material itself. Furthermore, smoke, dust, vapors and other gases caused by the induction heating of the material will also impair the accuracy of determining the radiant energy from the material. Finally, the optical pyrometer is limited to only estimating the surface temperature of the material and cannot accurately measure the internal temperatures within the heated material.
There have been, so far, relatively few successful control schemes to control the heating cycle for mass production heating systems. As disclosed in Balzer U.S. Pat. No. 4,618,125 and Mucha et al U.S. Pat. No. 4,728,761, it is possible to use eddy current analysis to determine the metallurgical characteristics of an object after it has been inductively heated and quench hardened to determine whether to accept or reject the hardened object based on comparisons to a preselected plan or pattern formed by an ideal hardened object. However, Both Balzer and Mucha are limited to post induction heating analysis of hardened objects.
Monitoring the temperature of an inductively heated object during quenching is disclosed in Pfaffmann U.S. Pat. No. 4,675,057. Pfaffmann employs the use of eddy current analysis to monitor and regulate the cooling rate of an object during quench hardening. However, the assignee has found that eddy current analysis is impaired during induction heating. The electromagnetic field produced by the induction heating coil can interfere with the analysis of eddy currents within the inductively heated workpiece. Indeed all prior patents have disclosed eddy current analysis either after the material has been hardened, as disclosed in Mucha, Balzer, and Mordwinkin U.S. Pat. Nos. 4,059,795 and 4,230,987, or during the cooling processes, as disclosed in Pfaffmann U.S. Pat. No. 4,675,057 and Spies U.S. Pat. No. 4,427,463.
Eddy current technology has not been successfully applied to in-process use in conjunction with induction heating. By using eddy current analysis, a non-destructive testing procedure of the workpieces can be continuously analyzed during the heating process to monitor and control the proper rate of heating and preferred maximum heating temperature for the workpiece and compare the data to preselected patterns and/or characteristics of acceptable workpieces. However, because of interference by the induction heating coil real time monitoring of induction heating by eddy current techniques has been unattainable.
In view of the present state of the art, assignee of the present application has been searching for an in-process system for monitoring and controlling the heating rates of workpieces during induction heating without requiring destructive testing of the workpiece.